1. Field of the Invention
This invention relates to an ophthalmologic instrument capable of displaying an alignment state when alignment of a subject's eye with an instrument body is adjusted to observe the subject's eye.
2. Description of the Prior Art
An ophthalmologic instrument is known in which an image of an anterior segment of a subject's eye is displayed at the middle of a display screen of a means for observing the eye and, in addition, an alignment level mark is displayed at the corner of the display screen simultaneously with the image of the anterior segment. The alignment level mark gradually changes in real time in accordance with the state of alignment, and thereby an operator can judge whether the alignment has been precisely adjusted or not.
There is also an ophthalmologic instrument, such as a non-contact type of tonometer, in which great accuracy in alignment is required. In this ophthalmologic instrument, rough alignment of the subject's eye with the instrument is first carried out by estimating it directly with the operator's eye or by observing an image of the subject's eye displayed on a display screen, and then fine alignment is carried out by checking an alignment level mark displayed on the display screen.
However, since the alignment level mark merely serves to inform the operator of an alignment state step by step, the operator disadvantageously cannot determine a direction in which the instrument body is moved with respect to the subject's eye in order to complete the alignment.
As a solution to this problem, an idea has been proposed that an index image is used to adjust alignment of the instrument body with the subject's eye in up, down, right, and left directions whereas an alignment level mark is used to determine a working distance between the instrument body and the subject's eye. The index image is a corneal reflection image formed by projecting a beam of index light for alignment onto a cornea of the subject's eye. The operator moves the instrument body in the up, down, right, and left directions so as to bring about a coincidence between a predetermined position on the display screen and a position of the index image superimposed on an image of the anterior segment of the subject's eye.
In this connection, there is a need to individually ascertain the index image and the alignment level mark for the alignment if they are individually displayed. In addition, in such an ophthalmologic instrument requiring the pinpoint accuracy as mentioned above, it is difficult to determine the working distance by the alignment level mark displayed at the corner of the display screen because the operator must carry out the alignment while watching the index image which is displayed in the middle of the display screen and is continuously trembling under the influence of the involuntary eye movement of the subject's eye.